Writing a finished song gives a satisfied feeling. It is important to know when you have finished your song.
Logical? Not for those guitarplayers who do not write often, as they lack the experience.
Songwriters who write all the time may work on several songs at the same time, less experienced writers may work on only one song at a time.

When you are not writing often changes are you work on something, get stuck after a while, leave it, and before you know it, months have passed by without finishing that songidea. When you come back to that idea later on, the inspiration may not be there anymore, and you may have forgotten what your idea was all about.
Try to keep at your song as much as you can, and try to finish it as much as you can. Try to have a least a melody of some kind and some sort of form. Do not worry about your lyrics not being good. Once you have an idea what the song will be about you can always finetune your lyrics, as long as you have a rough version you should be fine since finding the right words takes time!.

Finishing your songideas is important, at some point you will develop a feel for when your song is finished, or even get a feel for when you are on to something.
Experienced writers will know this. For people who write all the time I would also recommend to finish their songs, do not leave collections of half-finished ideas on your hard-drive. Nothing more frustrating than having to patch up half finished songs. Better to take them through right to the end.

Be not too critical about your songs, it takes time for the good ones to come through, and only when you have finished a song you will be able to tell whether it is good or bad one. On average you do need to write a lot: A collection of fifty songs may contain five good ones. Really? Yes, but you may just write for fun, so these standards I set out here do not really apply. Just write and enjoy what comes out of you. 

Hope to see you soon again,
Eddie